Not applicable.
Not applicable.
Not applicable.
This invention relates to a vibration damping silicone composition and particularly to a vibration damping silicone composition that has excellent long-term storage stability.
A large number of vibration damping compositions comprising a viscous fluid and solid powder have already been proposed. For example, Japanese Patent Application Publication Number Sho 63-308241 (308,241/1988) describes a vibration damping composition comprising a viscous fluid (e.g., silicone oil) and a solid powder such as silica powder, glass powder, or silicone resin powder. Japanese Patent Application Publication Number Hei 10-251517 (251,517/1998) describes a vibration damping composition comprising a silicone resin powder and a viscous fluid such as silicone oil. Japanese Patent Application Publication Number Hei 10-281202 (281,202/1998) describes a viscous fluid blend comprising silsesquioxane powder and non-surface-treated wet-process silica micropowder in silicone oil. Japanese Patent Application Publication Number Hei 11-182624 (182,624/1999) describes a viscous fluid blend of silsesquioxane powder, wet-process silica micropowder, and silanol-terminated dimethylsiloxane in silicone oil.
However, these compositions typically undergo changes when stored and/or used over extended periods of time, resulting in a deterioration in their vibration damping properties.
This invention relates to a vibration damping silicone composition comprising:
A. a silicone oil,
B. a silicone resin which is solid at room temperature, and
C. a surface-hydrophobic silica powder having a pH of greater than 6 and less than 9.
In a preferred composition there is provided
A. 100 parts by weight of silicone oil,
B. 10 to 300 parts by weight of a silicone resin powder which is solid at room temperature, and
C. 0.1 to 10 parts by weight of a surface-hydrophobic silica powder having a pH of greater than 6 and less than 9.
Component A, the Silicone oil, functions as the matrix component in which components B and C are dispersed. The silicone oil is an organopolysiloxane, comprising substantially of units of the formula Rxe2x80x22SiO2/2 and is a liquid at room temperature. The Rxe2x80x2 groups bonded to the silicon atoms may be the same or different and are typically monovalent hydrocarbon groups, for example alkyl groups such as methyl, ethyl, propyl and octyl groups; alkenyl groups such as vinyl, allyl, and butenyl groups and aryl groups such as phenyl and tolyl groups. Halogenated alkyl groups such as 3,3,3-trifluoropropyl may also be used. Small amounts of these hydrocarbon groups may be replaced by hydroxyl groups, or alkoxy groups such as methoxy, and ethoxy groups. Preferably most Rxe2x80x2 groups are alkyl groups, most preferably methyl groups. Alkyl groups are preferred because the composition in accordance with the present invention containing such silicone oils has a low degree of change of viscosity with respect to temperature and excellent storage stability.
Component A may be a linear, partially branched linear, branched, or cyclic. A linear structure is preferred. Component A should have a kinematic viscosity at 25xc2x0 C. of from 100 to 1,000,000 mm2/s, preferably from 500 to 500,000 mm2/s. When the kinematic viscosity at 25xc2x0 C. is less than 100 mm2/s, it is difficult to maintain components B and C in a uniformly dispersed state in component A. When the viscosity of component A exceeds 1,000,000 mm2/s, handling properties deteriorate, and components B and C are difficult to disperse in component A. Component A may be exemplified by trimethylsiloxy-endblocked dimethylpolysiloxanes, dimethylvinylsiloxy-endblocked dimethylpolysiloxanes, trimethylsiloxy-endblocked methyloctylpolysiloxanes, silanol-endblocked dimethylpolysiloxanes, and trimethylsiloxy-endblocked dimethylsiloxane-methylphenylsiloxane copolymers.
Component B, the Silicone resin powder, is a solid at room temperature and is utilised to enhance the vibration damping performance of the composition in accordance with the invention. Component B may be an organopolysiloxane containing RSiO3/2 units and/or SiO4/2 units in the main skeleton structure. Component B may also contain R2SiO2/2 units and/or R3SiO1/2 units. An organopolysilsesquioxane powder containing substantially only RSiO3/2 units is most preferred. Preferably R is a monovalent hydrocarbon group for example alkyl groups such as methyl, ethyl, and propyl; alkenyl groups such as vinyl, allyl, and butenyl; and aryl groups such as phenyl and tolyl. R may also be a halogenated alkyl group such as 3,3,3-trifluoropropyl. In addition, some of these groups may be substituted by hydroxyl groups or alkoxy groups such as methoxy and ethoxy groups. Alkyl groups are preferred as the resulting composition in accordance with the invention containing such resins has only a low degree of change of viscosity with respect to temperature and excellent storage stability. Each R may be the same or different, but it is preferred that at least 50 percent of the R groups are alkyl groups, most preferably methyl groups.
A small number of hydroxyl groups, halogen groups such as chlorine and/or alkoxy groups such as methoxy and ethoxy groups may be present at terminal ends in the molecular chain of the silicone resin powder. Component B may have an average particle size of from 0.1 to 100 xcexcm, preferably from 10 to 40 xcexcm. The shape of the particles may be spherical, oblate, or irregular. The amount of component B to be added per 100 parts by weight of component A may be in the range of from 10 to 300 parts by weight, preferably from 20 to 200 parts by weight. When the amount of component B is less than 10 parts by weight, the vibration damping properties of the composition according to the present invention tend to deteriorate, and when the amount of component B exceeds 300 parts by weight, the operating properties composition in accordance with the present invention deteriorate.
Component C, the surface-hydrophobic silica powder, functions in the composition according to the present invention as a means of improving both the storage stability and vibration damping performance of the composition. The pH of component C must be greater than 6 but less than 9 and is preferably in the range of from 6.5 to 8.5. Properties such as the thermal stability and storage stability undergo a decline at a pH of 6 or below and at a pH of 9 or above. For example component A may depolymerise in a more acidic environment than pH 6 or a more alkaline environment than pH 9. For the purposes of this invention, the pH values are all measured using a glass electrode pH meter, and each sample tested is an ethanol/water suspension of a surface-treated silica powder prepared by wetting 2 g of the surface-treated silica powder with 25 ml ethanol followed by dispersion of the ethanolic silica in 25 ml water whose pH has been adjusted to 7.0. Both wet-process and dry-process silicas may be used as the silica powder of component C. The silica used will generally have a BET specific surface area in the range of from 40 to 500 m2/g. Wet-process silica powder with a BET specific surface area in the range of from 50 to 400 m2/g is preferred. An organopolysiloxane with a low degree of polymerisation (DP) is generally used to surface treat the silica of component C to render it hydrophobic. The kinematic viscosity of the organopolysiloxane used to surface treat the silica is preferably from 0.5 to 100 mm2/s and the DP is preferably from 2 to about 90. Examples of the organopolysiloxane, which may be used to surface treat the silica of component C, include trimethylsiloxy- and dimethylsiloxy-endblocked dimethylpolysiloxanes, methylhydrogenpolysiloxanes, and dimethylsiloxane-methylhydrogensiloxane copolymers. The surface treatment of the silica powder with organopolysiloxane, to render it hydrophobic, may be carried out for example, by either:
i) adding the organopolysiloxane to the silica powder while the powder is being stirred and then heating the resulting mixture whilst continuing to stir; or
ii) spraying organopolysiloxane on to the silica powder while the powder is stirred and then heating the resulting mixture whilst continuing to stir.
This hydrophobing surface treatment may be carried out either in air or an inert atmosphere. The heating temperature will generally be in the range of from 100 to 500xc2x0 C. and preferably is in the range of from 150 to 450xc2x0 C. The treatment time will vary dependent on the treatment temperature, but will generally be from 10 minutes to 5 hours.
The extent of the surface treatment or hydrophobisation ratio may be expressed by the methanol value (M value). The M value is the lowest volume % of methanol in an aqueous methanol solution in which hydrophobic silica powder becomes sedimentable. That is, when a hydrophobic silica powder is introduced into a series of aqueous methanol solutions having different concentrations, the M value is the volume % of methanol in the least concentrated aqueous solution in which the silica powder has sedimented. The M value increases as the hydrophobisation ratio increases given that as a general matter it becomes increasingly difficult to achieve dispersion and sedimentation in dilute aqueous methanol solutions as the hydrophobisation ratio of the surface of the silica powder grows larger. The M value of component C is preferably in the range of from 20 to 50 and more preferably is in the range of from 25 to 40. The surface treatment will be inadequate and the storage stability will be impaired when the M value is less than 20, while the effect of the addition is reduced when the M value exceeds 50.
Component C is preferably added into the composition of the present invention in the range of from 0.1 to 10 parts by weight and more preferably 0.5 to 7.0 parts by weight per 100 parts by weight of component A. The vibration damping performance of the resulting composition declines when component C is present in the composition in an amount of less than 0.1 weight part, while the working properties of the resulting composition deteriorate when component C is present in the composition in an amount in excess of 10 parts by weight.
In addition to components A, B and C, compositions according to the present invention may include a variety of additives but only to an extent that the damping characteristics of the composition are not impaired. These may include, for example, inorganic micropowders such as glass micropowder, silica micropowder, clay, bentonite, diatomaceous earth, and quartz powder and organic resin micropowders such as acrylic resin micropowder, fluororesin micropowder, and phenolic resin micropowder. Other additives, which may be introduced into the composition of the present invention, include anti-oxidants, rust inhibitors, flame retardants, pigments and dyes. The composition is preferably in the form of a viscous liquid or semi-solid state at room temperature.
Compositions in accordance with the present invention containing components A, B and C may be prepared by combining and kneading the ingredients in a mixer such as, for example, a ball mill, a vibrating mill, a kneader-mixer, a screw extruder, a paddle mixer, a ribbon mixer, a Banbury mixer, a Ross mixer, a Henschel mixer, a flow jet mixer, a Hubbard mixer, or a roll mixer. Heating may be used during the mixing process, preferably at a temperature of from 30 to 200xc2x0 C. Whilst the pressure used during mixing may be atmospheric, mixing under a reduced pressure is preferred.
The composition in accordance with the present invention has the following characteristics: an excellent vibration damping performance, an excellent storage stability, and a low response to temperature variations. Because of these advantages, the composition may be used in shock absorbers prepared by filling elastic containers such as rubber bags or rubber cylinders with the composition. It is particularly suitable in shock absorbers used in electrical and electronic equipment in environments where there exist considerable temperature fluctuation, such as in compact disk players, compact disk changers, mini-disk players, and car navigation devices.